Spectrum geophysics

The frequency domain method is a method that uses the inductive properties of a continuous primary electromagnetic field to measure the conductivity (reciprocal of resistivity) of the material through which the field passes. This method has a long history in the mining industry for exploration of base metal deposits; however, it is now used for many environmental and engineering types of applications. Land-based frequency domain methods have 30 years of demonstrated use for the delineation of undocumented disposal, chemical waste and large buried metallic objects. Because the material of which the subsurface is composed controls the ability of the subsurface to carry an electrical current (or hold a secondary field), the conductivity of a material correlates well with grain size (and generally increases with decreasing grain size) these methods can be used to both identify lateral boundaries between different materials and lithology of the materials. Frequency domain methods are also highly sensitive to the chemistry of materials, and therefore can be used for many groundwater applications such as the delineation of shallow high-TDS and contaminant plumes (DNAPL and LNAPL) and the delineation of weathered, altered, or vertical water-bearing zones in bedrock. In addition, these methods are very sensitive to metallic material, and can be used to detect metallic pipes, USTs and drums. With the aid of a regularly spaced grid, these methods can be used to distinguish the boundaries between contrasting materials with a very high degree of resolution. Frequency domain methods are particularly useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively. For shallow applications Spectrum uses a Geonics EM-31terrain conductivity meter (EM-31); for deeper applications Spectrum uses a Geonics EM-34-3 terrain conductivity meter (EM-34).

EM-31

The EM-31 is an electromagnetic induction instrument that consists of two coils (transmitter and receiver) mounted on either end of a 12-foot-long plastic boom. During data acquisition, an alternating current is applied to `the transmitter coil, which sends a primary electromagnetic (EM) field into the ground. By Faraday’s Law of Induction, this primary field induces eddy currents in buried conductive material that is encountered, and these eddy currents generate a secondary magnetic field. This secondary magnetic field is measured at the receiver and compared to the primary field in terms of the component in phase with the primary field (in-phase) and the component out of phase with the primary magnetic field (the quadrature component). The quadrature component is converted to read conductivity in millimhos per meter. The in-phase component is set to read 0 in background materials, and is very sensitive to buried metal. The EM-31 is an instrument proven to be very effective for the delineation of lateral conductivity variations in soil to a maximum depth of 18 feet below the instrument height (in the vertical dipole mode), while the peak signal response depth is about 5 feet below the instrument height. Because the EM-31 was designed to be less affected by surface metallic material, this instrument is the best choice of geophysical equipment to detect buried features where small surface metallic material is present. The EM-31 is useful for the delineation of features such as oil brine pits, buried utilities, buried metallic and nonmetallic debris, buried tanks and drums, and chemical soil contamination.

During a typical EM-31 survey Spectrum establishes a base survey grid for navigation, and then the geophysics crew collects nearly-continuous (approximately 1or 2 feet apart) conductivity and in-phase data along equidistant parallel lines using a NavCom SF- 2050G StarFireTM GPS receiver for positional tracking. Once collected, the data are downloaded to a laptop, processed in the field and used to generate color-enhanced contour maps to assist in identifying anomalous areas of interest.

EM-34

The EM-34 is an electromagnetic induction instrument that consists of two separate coils – one transmitter coil and one receiver coil – separated by a known distance, where the greater the distance the greater the depth of investigation. During operation, the transmitter coil sends out a primary electromagnetic field that induces eddy currents in conductive subsurface materials. These eddy currents then generate a secondary magnetic field whose strength and phase shift are a function of the conductivity of the subsurface. At the receiver, a comparison between the primary magnetic field and the secondary magnetic field is made and a subsurface conductivity value is then calculated. The EM-34 requires two operators (one holding the transmitter and one holding the receiver) that traverse the ground as the data are acquired. Available separation distances are 10 meters (15 meter depth of exploration), 20 meters (30 meter depth of exploration), and 40 meters (60 meter depth of exploration). EM-34 data are acquired along parallel traverses at regular spacing to generate either profiles or a color contour map that represents conductivity variation for a specific depth range. This instrument has demonstrated effectiveness for groundwater exploration, delineation of subsurface conductive plumes, and for the lateral delineation of deep landfills.

The ability to collect both vertical dipole data (horizontal coils) and horizontal data (coplanar vertical coils) in a single EM-34 traverse provides a fast and effective method of obtaining conductivity data from two different depths for one coil separation; this feature is especially useful for groundwater applications and for locating near-vertical dikes because the response from the horizontal data is quite different from that of the vertical data in the presence of subsurface conductors and water-bearing fractures. In practice, the horizontal dipole (shallow) and vertical dipole (deeper) data are plotted together for a single profile in order to facilitate interpretation of features of interest.

The time domain method is a method that uses the inductive properties of a transient primary electromagnetic field to measure the ground response or resistivity of the material through which the field passes after the primary field is turned off. This method has its history in mineral exploration; however, it is now used for many environmental and engineering types of applications. For shallow applications Spectrum uses the Geonics EM-61 MK-2 high sensitivity metal detector (EM-61). This instrument is highly effective for the lateral delineation of metallic objects ranging in size from 3-inch military ordnance to 300-foot-long utilities, and can detect a buried 55-gallon drum under up to 9 feet of cover under favorable conditions. The EM-61 is particularly useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively. The EM-61 is very effective in open areas of industrial sites such as chemical plants, refineries, and tank farms or at demolished facility locations where no surface expression of the former buildings or utilities exist and utility location using standard methods is very limited.

For deeper applications Spectrum uses the Geonics EM-47 or EM-57 TEM system. These instruments measure the vertical electrical resistivity variation of subsurface materials, and are capable of imaging to significant depths. TEM methods are useful for the vertical delineation of geologic or hydrogeologic contacts in cases where the target is conductive and there is a strong contrast in electrical resistivity (or conductivity – its reciprocal) between one unit and the next. As such, TEM methods are effective for the delineation of sand/shale or sand/clay interfaces or the interface between freshwater and saltwater or freshwater and high-TDS-water aquifers. TEM methods using the EM-47/57 system are especially useful for sites where the space available is small (precludes the use of electrical resistivity), the target is deep (200 feet to 500 feet or more), or for sites where there is a large area to investigate and a limited budget, as many 1D measurements can be made quickly and relatively cost-effectively. In addition, lateral delineation of geologic or hydrogeologic features can be accomplished by stringing together several 1D TEM measurements in a linear fashion to create electrical cross sections.

EM-61

The EM-61 consists of two one-meter square coils, or two one-half-meter by one-meter coils, separated vertically by about one-half meter. The bottom coil contains both a transmitter and receiver, and the top coil contains just a receiver. During operation, the EM-61 transmitter generates short pulses of electromagnetic energy that travel downward and outward and have a primary field associated with them. This energy becomes “trapped” in conductive materials and causes a secondary magnetic field to be generated in these materials. Between pulses, each receiver measures the voltage of the decay of this secondary magnetic field at four fixed time gates; this voltage is proportional to the conductivity of the subsurface materials.

The EM-61 MK-2 was designed by Geonics to allow integration of GPS data with EM-61 data, and to allow better anomaly discrimination capability than the standard EM-61. The EM-61 MK-2 has four time gates whereas the standard EM-61 has two time gates; the first two time gates are earlier than those of the standard EM-61, the 3rd gate is equivalent to the standard EM-61 bottom coil response and the 4th time gate allows the user the option of recording a top coil response or a 4th gate response, the latter of which can be used to provide deeper information in areas where the EM-61 signal is not attenuated.

During a typical EM-61 survey Spectrum establishes a base survey grid for navigation, and then the geophysics crew collects nearly-continuous (approximately 1foot apart) data along equidistant parallel lines using either a NavCom SF- 2050G StarFireTM GPS receiver or a rectangular grid for positional tracking. Once collected, the data are downloaded to a laptop, processed in the field and used to generate color-enhanced contour maps of top coil, bottom coil, and/or differential (bottom coil subtracted from the top coil) data to assist in identifying anomalous areas of interest. Top or bottom coil data can be useful for identifying near-surface metallic objects; although, the top coil generally has a larger response than the bottom coil to deeply buried objects. The differential data is used to distinguish deeper targets such as buried reinforced vaults and reinforced concrete footings from shallow ones such as a vault lid or scrap metal. Utilization of the differential data allows for the suppression of near surface targets that might mask the response from deeper targets of interest. It is recommended that a minimum 10-foot buffer be established between the survey area and any metallic or metal-bearing surface cultural features such as cars, metallic signs, or aboveground piping, which could severely compromise the quality of the data. Reliable EM-61 data cannot be collected over areas covered with reinforced concrete.

EM-47/57

During a TEM central loop sounding (there are others such as the offset sounding) with the EM-47 or EM-57, the transmitter wire is laid out in a square loop on the ground – the dimension of the loop depends on the depth of investigation but can be anywhere from 5 to 100 meters - and the receiver coil is then placed in the center of the transmitter loop, where the center of the receiver coil falls precisely at the desired measurement location. The transmitter then sends out a current of one to several amps, which has an associated magnetic field, through the transmitter loop. Once this magnetic field has been established, the current is rapidly turned off. The magnetic field in the ground does not vanish immediately, but rather decays with time after the current is shut off. This decaying magnetic field is supported by induced (secondary) eddy currents in the subsurface. Immediately after the current in the loop is turned off, the eddy currents flow in a square ring in the subsurface just below the transmitter loop. As time increases, this secondary current ring descends into the ground in larger and larger dimension (but decreasing strength) square rings, much like a smoke ring dissipates. The rate at which the current dissipates and descends depends on the electrical conductivity of the subsurface materials. While the secondary currents dissipate, the decaying secondary magnetic field associated with these currents induces a decaying voltage in the receiver coil at the surface in the center of the transmitter loop (central loop sounding). The receiver measures this decaying voltage at 20 different time gates, representing increasing depths of investigation with increasing time. Because the rate of decay of the measured secondary magnetic field depends on the rate of dissipation of the induced secondary eddy currents established by the transmitter, which depends in turn on the electrical properties of the ground, the rate of decay of this measured secondary magnetic field measured at the receiver coil is representative of the electrical structure of the subsurface. As a result, a specific vertical resistivity layer model with depth gives rise to a unique measured decay curve.

During a typical EM-47/EM-57 survey, several transects are established, where individual 1D soundings are collected at regular station intervals, such as 5 or 10 meters up to 20 meters or more. Once collected, the data are downloaded and processed as discussed below.